1. Field of the Invention
The present invention relates generally to an electronic device, and more particularly to a light detection circuit employed in an electronic device, such as a portable transaction card, having secured information needing protection from unauthorized access.
2. Description of Related Art
Portable transaction cards are now in widespread use. These cards include, for example, cash and credit cards, stock certificates, passage, identification, and health insurance cards. Most portable transaction cards store information about respective bearers, such as card number, bearer""s name, expiration date, etc. The cards typically include a magnetic strip from which the information can be read. The information stored on these cards is of importance to their owners because loss of the information may result in unauthorized usage by others at the expense of the owners. Therefore, the information must be secured.
Recently, storage demands on portable transaction cards have increased. The traditional magnetic cards cannot satisfy the increasing storage demands. One solution is use of an advanced transaction card that stores more information than a magnetic card, i.e., IC (integrated circuit) cards or smart cards. The IC card is advantageous in physical integrity, as well as an enlarged storage capacity. Since the IC card stores more personal information, enhanced security features are needed for preventing unauthorized persons from accessing, stealing, or modifying information stored therein.
One method of securing information is by employing a light detection circuit on the IC card, from which a light detection signal is generated when an integrated circuit in the card is exposed to light. Once a light detection signal is generated, operations for processing and storing information from the card are inhibited. An example of such a circuit is found in U.S. Pat. No. 4,952,796, as shown in FIG. 1. The circuit of FIG. 1 has a diode 1 which is reverse-biased to a PMOS transistor 2 acting as a current load. Gate 3 of the PMOS transistor 2 is connected between a power supply voltage Vcc, a NMOS transistor 4, and coupled to a bias circuit 17. The bias circuit 17, having PMOS and NMOS transistors, 11 and 13, supplies a predetermined bias voltage to the PMOS transistor 2, turning the PMOS transistor 2 on. Thus, the PMOS transistor 2 retains an ON state while NMOS transistor 4 is in an OFF state.
When the diode 1 is not exposed to light, a negligible amount of a reverse saturation current (Is) flows through the diode 1. Therefore, the voltage level at node 29 is increased toward the power supply voltage. The output terminal 28 of an inverter including PMOS and NMOS transistors, 26 and 27, goes to low level, and output signal S from an inverter including PMOS and NMOS transistors, 30 and 31, goes to high level. Output signal SB from an inverter of PMOS and NMOS transistors, 32 and 33, is at low level.
When the diode 1 is exposed to light, a large amount of the reverse saturation current flows through the diode. If the reverse saturation current flows over a predetermined critical value (Im), the voltage level at the node 29 falls. Transistor 26 is turned on and transistor 27 is turned off, causing a voltage level at node 28 to be high and the output signal S to be low.
However, in the circuit shown in FIG. 1, generally the amount of the reverse saturation current varies due to manufacturing parameters and environmental temperature, which causes the critical value of the current, Im, to be uncertain. Further, the critical current value may be changed due to a current level of the power supply voltage.
The present invention solves the problem of variations in current Im described above. It is an object of the invention to provide a light detection circuit with a stable current during variations in environmental conditions.
It is another object of the invention to provide a light detection circuit, used in a personal transaction card such an integrated circuit card, which has a stabilized operation when environmental influences such as manufacturing parameters, temperature, and power supply voltage variations are present.
To accomplish those objects, according to a preferred embodiment of the invention, one feature of the present light detection circuit includes a first comparator for receiving a first signal having a first voltage and a second signal having a second voltage, the first voltage is discharged from a first predetermined voltage and the second voltage is discharged from a second predetermined voltage upon application of a power supply voltage, the second signal being responsive to light. The light detection circuit further includes a second comparator for receiving the second voltage and a reference voltage, and a circuit for charging the first and second voltages up to the first and second predetermined voltages in response to an active output signal from the second comparator.
An output signal of the first comparator is stored in a latch circuit and output from the latch circuit as a light detection signal in response to an output signal of the second comparator.
Alternatively, an electronic device having a light detection circuit for detecting the presence of light according to the present invention includes a constant voltage supply connected to a power supply node, a sensing circuit including an element responsive to light and connected to the power supply node, a reference voltage circuit connected to the power supply node, a first circuit for comparing an output of the sensing circuit with an output of the constant voltage supply and generating an output which determines a current sensing state, and a second circuit for comparing the sensing circuit output with a reference voltage and generating an output which controls the constant voltage supply, the sensing circuit, and the output of the first circuit. The light detection circuit further includes, a latch circuit for storing the output of the first circuit and generating a light detection signal in response to the output of the second circuit.
Another light detection circuit according to the present invention includes a constant voltage supply connected to a power supply node, a sensing circuit including an element responsive to light and connected to the power supply node, a first circuit for comparing an output of the sensing circuit with an output of the constant voltage supply and generating an output which determines a current sensing state, a latch circuit for storing the output of the first circuit, and a second circuit for comparing the sensing circuit output with a reference voltage and generating an output which controls the constant voltage supply, the sensing circuit, and the latch circuit. The latch circuit outputs a light detection signal in response to the output of the second circuit. The light detection circuit can be employed in an electronic device such as and integrated circuit card.
The constant voltage supply includes a PMOS transistor disposed between the power supply node and a constant voltage supply node, a capacitor disposed between the voltage supply node and a substrate voltage, and a diode disposed between the substrate voltage and the voltage supply node.
These and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be used in connection with the accompanying drawings.